1. Field of the invention
The present invention finds an application in telecommunications and in particular in a mobile telephone network.
2. Description of the prior art
In a mobile telephone network the clocks of the various base transceiver stations must be synchronized and it is often necessary to provide means for keeping them synchronized despite the tendency of the clock frequency to drift with time. The invention applies, with advantage in the situation where the stations exchange information in the form of packets transmitted via an asynchronous transmission network. In the context of calls set up between network subscribers, the information is the human voice or data, for example. A station generally sends the packets at constant intervals. The asynchronous network imposes transit delays on them, however, and these delays are subject to random fluctuations.
One method of securing phase agreement is the Synchronous Residual Time Stamp (SRTS) method. It uses a reference clock whose signals are received by the base transceiver stations via transmission means imposing constant transit delays on the signals. It has the disadvantage that the aforementioned means must connect the clock to all the stations, which makes it costly to implement.
This is why a second prior art method has been defined to maintain phase agreement once it has been established. This method has the advantage of using only the asynchronous network for this purpose, which has to be used in any case to transmit the packets conveying information. It has the further advantage that the packets are the only signals injected into the network. This process is referred to as xe2x80x9cadaptive synchronizationxe2x80x9d. In substance, it continuously accumulates the intervals between the packets received at the output of the asynchronous network and compares the result with the sum of the same number of theoretical intervals defined by a local clock. The result of this comparison constitutes an error signal, i.e. a signal that is used to define any increase required in the clock frequency, variation of the signal modifying the clock frequency and variation of the clock frequency modifying the signal by means of a feedback loop. This method therefore slaves the frequency to that of a reference clock which times transmission of packets. Any slow drift of the reference clock frequency affects the local clock. The method has the disadvantage that the local clock frequency is subject to low-frequency jitter referred to as xe2x80x9cwonderxe2x80x9d.
The above first and second prior art methods are described in more detail in the ITU-T standard 1.363.1, in paragraphs 2.5.2.2.2 and 2.5.2.2.1, respectively.
The present invention concerns adjusting the frequency of a local clock on the basis of that of a reference clock via an asynchronous transmission network and one object of the present invention is to achieve this by means of fast, accurate and stable (i.e. jitter-free) control at limited cost.
To this end, the present invention consists in a method of controlling a frequency via an asynchronous transmission network, in which method signals received at the output of the asynchronous transmission network have been clocked at a reference frequency and transmitted by the network with respective varying transit delays and the frequency of a local clock is slaved to the reference frequency by means of an error signal formed from selected received signals with minimal transit delays.
In the description of this invention a signal is referred to as xe2x80x9cfastxe2x80x9d and its transit delay is referred to as xe2x80x9cminimumxe2x80x9d if the delay is the shortest or practically the shortest of the transit delays of the signals received at the output of the asynchronous network during a period including reception of the fast signal. Several such periods can be defined and a minimum transit delay con be found in each period. Such periods are referred to hereinafter as xe2x80x9csearch periodsxe2x80x9d.
The error signal is typically formed from respective time offsets associated with the received signals. Each offset occurs between two corresponding instants, one of which belongs to a received series made up of instants respectively marked by said received signals and the other to a theoretical series defined by said local clock, this instant from the received sequence being the one marked by the signal associated with the offset.
In the received sequence, the marked instants form a succession at partly random intervals. However, they typically formed a succession at a constant interval in the sequence that the signals formed when they were transmitted, the interval being defined on the basis of the reference frequency. The theoretical sequence defined by the local clock then also has constant intervals if the frequency of the clock is itself constant.
Various means can be employed to select the fast signals. For example, each signal can have a time stamp indicating the time at which it was injected into the transmission network, and that time is compared with the time defined by the local clock for the reception of that signal and this comparison provides a basis for selecting the fast signals, at least approximately. However, this has the disadvantage that adding the time stamps increases the cost of the method. This is why, in the context of the present invention, a received signal is preferably selected as a fast signal when the offset associated with that signal has a minimum value in a group of such offsets. The group is formed by the offsets which are associated at the instants in the received sequence which are within one of the search periods mentioned above.
The invention is described in more detail hereinafter with reference to the accompanying diagrammatic drawings, with an indication, by way of example, of how it can be put into effect.